We elucidate the interplay between the ion-water and water-water interactions in determining the structures of halide ion-water clusters using infrared spectroscopy, interpreted with ab initio theory. Vibrational predissociation spectra of the X-.(H2O)(2). Ar-m (X = F, Cl, Br, I) clusters in the OH stretching region (2300-3800 cm(-1)) reveal a strongly halide-dependent pattern of bands. These spectra encode the incremental weakening of the interaction between the water molecules with the lighter halides, finally leading to their complete dissociation in the fluoride complex. A consequence of this is that the F-.(H2O)(2) cluster is likely to be a floppy system with high amplitude zero point motion, in contrast to the pseudo-rigid behavior of the other halide hydrates.